This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The principal goal of this study is to understand the functional organization of neurons in layers 2/3 of primary visual cortex (V1). We are examining the organization of cells with defined response properties, both in their relation to larger compartments like cytochrome-oxidase blobs and orientation maps, as well as in their fine-scale arrangement. These issues have been studied in the past with two approaches: electrophysiological recordings, which yield functional characterizations of single neurons, and intrinsic signal imaging, which provides an overview of functional maps in cortex but with very low resolution (less than 100 m). The key aspect of our present work is that we are using a new approach, calcium imaging with two-photon laser-scanning microscopy, which bridges the gap between single-unit electrophysiology and conventional optical imaging. The technique gives us single cell resolution from an essentially complete sample of cells in a given patch of cortex, and, to our knowledge, this is the first time it has been used to study cortical microarchitecture in primates. Our initial results have revealed an intriguing and very clear clustering of color-selective cells into functional units, most likely related to cytochrome-oxidase blobs. Within these color clusters we find an even more detailed organization, with blue ON cells segregated from blue OFF cells, and with groups of blue/yellow cells arranged in clusters distinct from red/green cells. These results could not have been predicted from earlier work, and they provide a framework for resolving some long-standing debates in the visual physiology literature.